<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds --> <HTML> <HEAD> <TITLE>10.5 Portable conditional compilation</TITLE> <META NAME="description" CONTENT="10.5 Portable conditional compilation"> <META NAME="keywords" CONTENT="lpg"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <LINK REL=STYLESHEET HREF="lpg.css"> </HEAD> <BODY LANG="EN"> <A NAME="tex2html2177" HREF="node149.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="/icons//next_motif.gif"></A> <A NAME="tex2html2175" HREF="node136.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="/icons//up_motif.gif"></A> <A NAME="tex2html2169" HREF="node147.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="/icons//previous_motif.gif"></A> <A NAME="tex2html2179" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="/icons//contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html2178" HREF="node149.html">10.6 Additional Comments</A> <B>Up:</B> <A NAME="tex2html2176" HREF="node136.html">10 Porting Applications to </A> <B> Previous:</B> <A NAME="tex2html2170" HREF="node147.html">10.4.3 Process control under </A> <BR> <P> <H1><A NAME="SECTION001150000000000000000">10.5 Portable conditional compilation</A></H1> <P> If you need to make modifications to existing code in order to port it to Linux, you may need to use <TT>ifdef</TT>...<TT>endif</TT> pairs to surround parts of Linux-specific code--or, for that matter, code corresponding to other implementations. No real standard for selecting portions of code to be compiled based on the operating system exists, but many programs use a convention such as defining <TT>SVR4</TT> for System V code, <TT>BSD</TT> for BSD code, and <TT>linux</TT> for Linux-specific code. <P> The GNU C library used by Linux allows you to turn on various features of the library by defining various macros at compile time. These are: <UL><LI> <TT>__STRICT_ANSI__</TT>: For ANSI C features only<LI> <TT>_POSIX_SOURCE</TT>: For POSIX.1 features<LI> <TT>_POSIX_C_SOURCE</TT>: If defined as 1, POSIX.1 features; if defined as 2, POSIX.2 features.<LI> <TT>_BSD_SOURCE</TT>: ANSI, POSIX, and BSD features.<LI> <TT>_SVID_SOURCE</TT>: ANSI, POSIX, and System V features.<LI> <TT>_GNU_SOURCE</TT>: ANSI, POSIX, BSD, SVID, and GNU extensions. This is the default if none of the above are defined. </UL> <P> If you define <TT>_BSD_SOURCE</TT> yourself, the additional definition <TT>_FAVOR_BSD</TT> will be defined for the library. This will cause BSD behavior for certain things to be selected over POSIX or SVR4. For example, if <TT>_FAVOR_BSD</TT> is defined, <EM>setjmp</EM> and <EM>longjmp</EM> will save and restore the signal mask, and <EM>getpgrp</EM> will accept a PID argument. Note that you must still link against <TT>libbsd</TT> to get BSD-like behavior for the features mentioned earlier in this paper. <P> Under Linux, <TT>gcc</TT> defines a number of macros automatically which you can use in your program. These are: <UL><LI> <TT>__GNUC__</TT> (major GNU C version, e.g., 2)<LI> <TT>__GNUC_MINOR__</TT> (minor GNU C version, e.g., 5)<LI> <TT>unix</TT><LI> <TT>i386</TT><LI> <TT>linux</TT><LI> <TT>__unix__</TT><LI> <TT>__i386__</TT><LI> <TT>__linux__</TT><LI> <TT>__unix</TT><LI> <TT>__i386</TT><LI> <TT>__linux</TT> </UL> Many programs use <PRE> #ifdef linux</PRE> to surround Linux-specific code. Using these compile-time macros you can easily adapt existing code to include or exclude changes necessary to port the program to Linux, Note that because Linux supports more System V-like features in general, the best code base to start from with a program written for both System V and BSD is probably the System V version. Alternately, you can start from the BSD base and link against <TT>libbsd</TT>. <P> <HR><A NAME="tex2html2177" HREF="node149.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="/icons//next_motif.gif"></A> <A NAME="tex2html2175" HREF="node136.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="/icons//up_motif.gif"></A> <A NAME="tex2html2169" HREF="node147.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="/icons//previous_motif.gif"></A> <A NAME="tex2html2179" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="/icons//contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html2178" HREF="node149.html">10.6 Additional Comments</A> <B>Up:</B> <A NAME="tex2html2176" HREF="node136.html">10 Porting Applications to </A> <B> Previous:</B> <A NAME="tex2html2170" HREF="node147.html">10.4.3 Process control under </A> <P><ADDRESS> <I>Converted on: <BR> Fri Mar 29 14:43:04 EST 1996</I> </ADDRESS> </BODY> </HTML>